This invention relates to screw compressors and more particularly to means for improving the capacity control characteristics of a screw compressor having a slide valve for controlling capacity.
As a means for effecting control of the capacity of a screw compressor, a so-called slide valve system is known in which the rotor casing for housing screw rotors is constructed in a manner to enable a portion thereof to move axially, so that a portion of the gas in the working chamber can be passed in a bypass flow to the suction side of the compressor when capacity control is effected.
This capacity control system is described, for example, in U.S. Pat. Nos. 3,885,402 (Harold W. Moody, Jr. et al), 3,913,346 (Harold W. Moody, Jr. et al), 3,936,239 (David N. Show) and 4,062,199 (Keisuke Kawahara et al). A screw compressor having a slide valve will be outlined. In a screw compressor of this type, a pair of male and female rotors is housed in a casing and rotatably journalled by bearings mounted on suction cover and a discharge cover provided to the casing. Gas is introduced into the casing through a suction port and discharged through a discharge port after being compressed. A capacity controlling slide valve is mounted in the casing and cooperates with the male and female rotors, casing, suction cover and discharge cover to define a working chamber for compressing the sucked gas. The slide valve is capable of moving axially thereof, and when the slide valve moves axially, a portion of the gas in the working chamber is bypassed to the suction side of the compressor through a bypass passage formed between the casing and slide valve and at the same time initiaton of compression of the gas is delayed to thereby effect capacity control of the compressor. The slide valve is moved by a hydraulic piston through a slide valve drive shaft. The slide valve and the discharge cover are formed with a radial discharge port and an axial discharge port respectively.
In this type of screw compressor of the prior art, the axial and radial discharge ports are of a size such that they are compatible with an internal volume ratio V.sub.iF for operation at full load (the volume ratio of the theoretical maximum volume of operating chamber after gas is sealed to the theoretical minimum volume of operating chamber immediately before gas is discharged). Because of this design characteristic, the screw compressor would have the disadvantage that the size of the discharge ports does not match the operating pressure ratio of the compressor when the latter operates at partial load, thereby reducing the capacity control characteristics of the compressor.
In order to improve capacity control characteristics, two proposals have been made. One is to match the size of the radial discharge port with the internal volume ratio V.sub.iF for operation at full load while the axial discharge port has its size made smaller than that of the radial discharge port, so that the compressor will have a built-in volume ratio for operation at partial load V.sub.iP =V.sub.iF. The other is to use a separate special slidable valve for increasing the size of the axial discharge port, such as the one described in U.S. Pat. No. 3,314,597 (Laulitz Benedictus Schibbye), for example.
In the former, if the axial discharge port had a size such that the built-in volume ratio V.sub.iP for operation at partial load is equal to the internal volume ratio V.sub.iF for operation at full load when the slide valve is fully close, capacity control characteristics would be greater improved when the compressor operates at high capacity with small slide valve opening. However, when the compressor operates at medium capacity with a medium slide valve opening, supercompression would occur and the capacity control characteristics would rather be reduced because V.sub.iP &gt;V.sub.iF.
The latter uses a special slidable valve, in addition to the slide valve, that can be actuated by means mounted outside the casing when the compressor operates under capacity control, while the axial and radial discharge ports have a size such that they are compatible with the internal volume ratio V.sub.iF for operation at full load. The special slidable valve is operative to cause a portion of the working fluid to flow to the high pressure passage side, but the mechanism for causing this flow is complex. Moreover, no satisfactory solution has been provided to meet the requirements that the built-in volume ratio V.sub.iP for operation at partial load should not become too low and that no supercompression should occur when slide valve opening is increased.